1. Field of the Invention
The present invention is directed to a method for ultrasound imaging, and in particular to such a method for producing an image of plane of a section of an examination subject having a surface-proximate region having a non-uniform speed of sound distribution.
2. Description of the Prior Art
An ultrasound imaging method is described in European application 0 256 481, corresponding to U.S. Pat. No. 4,817,614, which includes scanning the plane of the section to be imaged line-by-line both in an adaption phase and in an imaging phase. In the adaption phase, an image is not generated, but instead the signal delays of the signals received by the elementary transducers are compared to signal delays which would be expected given a uniform speed of sound distribution in the examination subject, and deviations from the expected values are calculated in aperture of the transducer array. In the imaging phase, correction values for reception focussing along an individual scan line are formed from the deviating values for a respective plurality of scan lines. The delay times with which the echo signals are delayed by the elementary transducers are modified dependent on the correction values in the imaging phase given focussed reception, so that image disturbances due to the non-uniform speed of sound distribution are compensated. This known method is referred to as the "principle of the adaptive antenna" or as the "principle of the adaptive aperture change."
In the adaption phase, the disturbing effects on the signal in the subcutaneous fatty tissue are measured once in the section plane of interest. In the following imaging phase, these disturbing effects are compensated in each scan in the sense of the adaptive antenna. The measurement of the disturbing effects caused by the non-uniform speed of sound distribution, which is implemented first, ensues exclusively on the basis of the reflected echo signals from the body of the patient. A cross-correlation of neighboring elementary transducers of the ultrasound array is formed. Variations in the speed of sound in the examination path of the patient immediately preceding the ultrasound array are determined by deviations in the maximum of the cross-correlation function compared to the anticipated value of the case of uniform distribution.
In order to avoid image artifacts, the following averaging possibilities are available.
The echo signals of a sufficiently large depth region of the examination subject can be used for the correlation. The depth region is selected so that the paths to the points at the boundaries of the depth region, in the region of the transducer-proximate disturbing layer, do not proceed significantly differently. The thinner the disturbing layer is assumed to be, the larger the depth regions can be selected. Conversely, the depth regions must be selected smaller if the thickness of the disturbing layer is increased.
Further noise suppression can be achieved by averaging the measured values of a plurality of neighboring scan lines. The aforementioned path differences must also be taken into consideration in this technique.
Another possibility for acquiring a number of different data sets independent of one another is that of measuring over a longer time span and averaging the data. A disadvantage is that differences in the echo signals will arise due to motion in the interior of the body over this longer time span.
In addition to the immediately preceding technique, an intentional, slight tilting of the transducer array can be used to create the independent data sets.
All of these averaging methods permit compensation of disturbing effects if the inhomogeneities in the surface-proximate region of the examination subject extend to a depth of approximately 10 mm. Given thicker disturbing layers in front of the ultrasound array, the differences in the transit times to the transducers become too significant to make use of the above techniques. The fluctuation of the speed of sound in the disturbing layer acts differently on the signal of an elementary transducer dependent on the position of the transducer within the active aperture.